Test case: replace type with a more generic one without major change.

test_crates/type_alias_over_more_generic_underlying/new/Cargo.toml

@@ -0,0 +1,7 @@
+[package]
+publish = false
+name = "type_alias_over_more_generic_underlying"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]

test_crates/type_alias_over_more_generic_underlying/new/src/lib.rs

@@ -0,0 +1,78 @@
+//! We're going to replace the original type (named like `OriginalType`) with two types:
+//! - `pub struct NewType<A, B = ()>`
+//! - `pub type OriginalType<A> = NewType<A>`
+//!
+//! This is not semver-major, since the type alias has:
+//! - the same name
+//! - the same fields/variants
+//! - the same constructibility and pattern-matching behavior.
+
+pub struct NewStructOnlyPrivateFields<A> {
+    _marker: std::marker::PhantomData<A>,
+}
+
+pub type OriginalStructOnlyPrivateFields = NewStructOnlyPrivateFields<()>;
+
+// ---
+
+pub struct NewStructGeneric<A, B = ()> {
+    _marker: std::marker::PhantomData<(A, B)>,
+}
+
+pub type OriginalStructGeneric<A> = NewStructGeneric<A>;
+
+// ---
+
+pub struct NewStructMixOfFields<A> {
+    _marker: std::marker::PhantomData<A>,
+    pub x: i64,
+}
+
+pub type OriginalStructMixOfFields = NewStructMixOfFields<()>;
+
+// ---
+
+pub struct NewStructConstructible<T> {
+    pub x: T,
+}
+
+pub type OriginalStructConstructible = NewStructConstructible<i64>;
+
+// ---
+
+pub struct NewStructEmpty<A> {
+    _marker: std::marker::PhantomData<A>,
+}
+
+pub type OriginalStructEmpty = NewStructEmpty<()>;
+
+// ---
+
+#[non_exhaustive]
+pub struct NewStructConstructibleNonexhaustive<A> {
+    pub x: A,
+}
+
+pub type OriginalStructConstructibleNonexhaustive = NewStructConstructibleNonexhaustive<i64>;
+
+// ---
+
+#[non_exhaustive]
+pub struct NewStructEmptyNonexhaustive<A> {
+    pub x: A,
+}
+
+pub type OriginalStructEmptyNonexhaustive = NewStructEmptyNonexhaustive<i64>;
+
+// ---
+// The enum case is interesting: to add a generic parameter, we need to use it somewhere.
+// We'll need to add a variant, so the original enum must have been non-exhaustive
+// to avoid an immediate major breaking change.
+
+#[non_exhaustive]
+pub enum NewEnumNonexhaustive<A> {
+    First,
+    Second(A),
+}
+
+pub type OriginalEnumNonexhaustive = NewEnumNonexhaustive<()>;

test_crates/type_alias_over_more_generic_underlying/old/Cargo.toml

@@ -0,0 +1,7 @@
+[package]
+publish = false
+name = "type_alias_over_more_generic_underlying"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]

test_crates/type_alias_over_more_generic_underlying/old/src/lib.rs

@@ -0,0 +1,46 @@
+//! We're going to replace the original type (named like `OriginalType`) with two types:
+//! - `pub struct NewType<A, B = ()>`
+//! - `pub type OriginalType<A> = NewType<A>`
+//!
+//! This is not semver-major, since the type alias has:
+//! - the same name
+//! - the same fields
+//! - the same constructibility and pattern-matching behavior.
+
+pub struct OriginalStructOnlyPrivateFields {
+    _marker: std::marker::PhantomData<()>,
+}
+
+pub struct OriginalStructGeneric<A> {
+    _marker: std::marker::PhantomData<A>,
+}
+
+pub struct OriginalStructMixOfFields {
+    _marker: std::marker::PhantomData<()>,
+    pub x: i64,
+}
+
+pub struct OriginalStructConstructible {
+    pub x: i64,
+}
+
+pub struct OriginalStructEmpty {}
+
+#[non_exhaustive]
+pub struct OriginalStructConstructibleNonexhaustive {
+    pub x: i64,
+}
+
+#[non_exhaustive]
+pub struct OriginalStructEmptyNonexhaustive {
+    pub x: i64,
+}
+
+// The enum case is interesting: to add a generic parameter, we need to use it somewhere.
+// We'll need to add a variant, so the original enum must have been non-exhaustive
+// to avoid an immediate major breaking change.
+
+#[non_exhaustive]
+pub enum OriginalEnumNonexhaustive {
+    First,
+}